Capacitors are the basic energy storage devices in random access memory devices, such as dynamic random access memory (DRAM) devices and ferroelectric memory (FERAM) devices. They consist of two conductors, such as parallel metal or polysilicon plates, which act as the electrodes (i.e., the storage node electrode and the cell plate capacitor electrode), insulated from each other by a dielectric material.
The continuous shrinkage of microelectronic devices such as capacitors over the years has led to a situation where the materials traditionally used in integrated circuit technology are approaching their performance limits. Silicon (i.e., doped polysilicon) has generally been the substrate of choice, and silicon dioxide (SiO2) has frequently been used as the dielectric material to construct microelectronic devices. However, when the SiO2 layer is thinned to 1 nm (i.e., a thickness of only 4 or 5 molecules), as is desired in the newest microelectronic devices, the layer no longer effectively performs as an insulator due to the tunneling current running through it. This SiO2 thin layer deficiency has lead to the search for improved dielectric materials.
High quality dielectric layers containing Group IIA metal titanates such as SrTiO3, BaTiO3, and (Ba1-xSrx)TiO3 are of interest to the semiconductor industry as they exhibit higher permitivities than do dielectric layers containing SiO2. Consequently, the semiconductor industry has been extensively evaluating strontium, barium, and titanium precursor compounds that can be used in vapor deposition processes.
Chemical vapor deposition (CVD) is a continuous, single step vapor deposition process that can be used to deposit dielectric films (i.e., layers) having excellent conformality and is therefore of significant interest in making strontium and barium titanate thin films. In CVD, excellent conformality is achieved when the process is carried out at a temperature low enough that the surface reactions are the rate-limiting step in the film growth. At higher temperatures the precursor compound transformation becomes the limiting factor, causing a degradation of the conformality.
Atomic layer deposition (ALD) is a more sophisticated vapor deposition process capable of forming even higher quality dielectric layers due to the self-limiting film growth and the optimum control of atomic-level thickness and film uniformity. Using the ALD process, several sequential process cycles are employed to deposit the layer on the substrate one monolayer at a time per cycle until the desired layer thickness is achieved. For each cycle of the ALD process, vapors of one or more precursor compounds are pulsed into the deposition chamber and are chemisorbed onto the substrate. Typically, one or more vaporized reaction gases (e.g., water vapor) are pulsed into the deposition chamber to react with the chemisorbed precursor compound(s) and cause the deposition of the desired layer onto the substrate. With the ALD process, more reactive precursors can be used, without the problem of gas-phase reactions, resulting in lower temperature requirements at the substrate.
Vehkamäki et al., “Growth of SrTiO3 and BaTiO3 Thin Films by Atomic Layer Deposition,” Electrochemical and Solid-State Letters, 2(10):504–506 (1999) describe thin films of SrTiO3 and BaTiO3 deposited by ALD processes making use of a novel class of strontium and barium precursors, i.e., their cyclopentadienyl compounds, together with titanium tetraisopropoxide and water. Prior to their discovery, Vehkamäki et al. state that the selection of strontium and barium precursor compounds has been limited to their β-diketonate compounds that do not react with water or oxygen at temperatures low enough for the self-limiting growth mechanism of ALD processes.
The search continues for sufficiently volatile Group IIA metal precursor compounds, especially strontium and/or barium precursor compounds, to employ successfully in vapor deposition processes, particularly ALD processes, to form, for example, strontium titanate, barium titanate, and barium-strontium titanate dielectric layers.